Calculators
Process control math with US and metric units, formulas and worked examples.
State Point Clarifier and RAS Control Tool
Plot the solids flux curve, overflow and underflow lines from settling tests or SVI, see whether your clarifiers are underloaded, critical or overloaded, and get a suggested RAS range.
Process control
SRT and MCRT Calculator
Solids retention time is the average number of days solids stay in the system. It controls nitrification, floc quality and sludge production.
Sludge Age Calculator (Gould)
Gould sludge age compares the solids under aeration with the suspended solids entering the aeration basin each day. It does not need wasting data, but it is less directly tied to biology than SRT.
F/M Ratio Calculator
F/M compares the daily BOD load with the biomass under aeration. Conventional plants often run about 0.05 to 0.4 lb BOD/lb MLVSS/d.
SVI Calculator
SVI is the volume in mL occupied by 1 gram of mixed liquor after 30 minutes of settling. 80 to 150 mL/g is typical; above 150 means slow settling.
RAS Flow Calculator
Three ways to estimate the RAS flow you need. When they agree, you can be confident. When they do not, the difference tells you something.
WAS Rate Calculator
Calculate how much sludge to waste each day to hold a target SRT.
MLSS Inventory Calculator
The solids inventory is the starting point for SRT, F/M and wasting calculations.
Clarifiers
Clarifier Surface Overflow Rate Calculator
SOR is flow divided by clarifier surface area. It controls clarification.
Solids Loading Rate Calculator
SLR is the mass of solids applied to a clarifier per unit area. It controls thickening.
Weir Overflow Rate Calculator
Weir overflow rate is flow per foot of weir length.
Loading
Aeration Basin Loading Calculator
Volumetric loading compares the BOD load with basin volume. Conventional activated sludge is often 20 to 40 lb BOD/1,000 ft³/d; extended aeration 10 to 25.
Pounds Loading Calculator
The pounds formula converts a flow and concentration into a mass per day.
BOD Loading Calculator
Calculate the daily BOD load and the approximate population equivalent.
Ammonia Loading Calculator
Calculate plant ammonia load, including a return stream like centrate, and the oxygen needed to nitrify it.
Aeration
Chemistry
Alkalinity for Nitrification Calculator
Estimate residual alkalinity after nitrification and the chemical needed to reach a target.
Chemical Feed Calculator
Convert a target dose into pounds per day and gallons per day of product.
Chlorine Dose Calculator
Calculate chlorine feed from demand and residual, as gas or hypochlorite solution.
Hydraulics
Detention Time Calculator
Detention time is tank volume divided by flow.
Pump Calculations
Three common pump calculations: flow from a wet well test, total dynamic head, and horsepower.
Unit Conversions
Flow, volume, mass, pressure, concentration, loading, power and temperature.
Lab
Oxygen Uptake Rate (OUR) Calculator
OUR measures how fast the biomass uses oxygen. A sudden drop can signal toxicity.
SOUR Calculator
SOUR is OUR per gram of volatile solids. It compares activity between days and plants.
Solids
Polymer Dose Calculator
Calculate polymer use in lb active per dry ton and the neat polymer feed needed for a target dose.
Volatile Solids Reduction Calculator
Calculate digester volatile solids reduction two ways.
Instruments and Controls
4-20 mA Signal Converter
Most plant transmitters send a 4 to 20 mA signal where 4 mA is the bottom of the range (LRV) and 20 mA is the top (URV). Use this to check what the PLC or SCADA should show for a current you measure, or what current to expect for a process value. It also flags currents that NAMUR NE43 treats as a failure.
Instrument Accuracy and Calibration Check
An accuracy spec means very different things depending on whether it is a percent of span, of reading, or of the sensor upper limit. The first tool shows the real error band at the reading you care about. The second records a 5-point check (0, 25, 50, 75 and 100% of range) and grades the as-found and as-left results against your tolerance.
Flume and Weir Flow Calculator
Open channel flow meters measure head and convert it to flow with a fixed equation for the primary device. Use this to check the flow your level transmitter or flow meter shows, or to build a check table. US units: head in feet, flow in cubic feet per second, gpm and MGD. The equation used and its valid range are shown with each result.
Mag Meter Velocity Calculator
A mag meter measures velocity and multiplies by the pipe area. Too slow and accuracy suffers and coatings build up; too fast and you get head loss and wear. Check the velocity at your normal and minimum flows, or find the size that gives a target velocity.
Tank Level, Volume and Hydrostatic Pressure
Chemical, sludge and wet well levels are often measured as pressure and then shown as a level or volume. The first tool turns a level into gallons for common tank shapes. The second turns a pressure reading into a level, corrected for specific gravity and the sensor mounting height.
Pressure Unit Converter
Gauges, transmitters and pump curves use different pressure units. Enter a value once and see it in every common unit, including inches of water at both 39.2 °F and 68 °F, since transmitters differ.
Pump Affinity Laws and VFD Speed
When a VFD slows a centrifugal pump, flow drops with speed, head with speed squared and power with speed cubed. That is why VFDs save energy. It is also why a pump slowed too far cannot lift against static head and just churns.
Three-Phase Motor Power Calculator
Clamp-on amps and a voltage reading tell you a lot about a motor. Use these to estimate real power, shaft horsepower and yearly energy cost, or to estimate the current a motor of a given size should draw.
DO Saturation and Sensor Check
A DO analyzer is calibrated against the oxygen that water can hold at saturation, which falls with temperature and elevation. This finds the saturation value for your conditions, shows a reading as percent saturation, and compares an online analyzer with a Winkler or calibrated handheld reading.
pH Electrode Slope and Buffer Calculator
A pH electrode makes about 59 mV per pH unit at 25 °C, less when cold and more when warm. A healthy electrode is close to that ideal. Use these tools to correct buffer values for temperature, grade a two-point calibration by its slope and offset, and see the theoretical slope at your sample temperature.